X.509 : A standard defining the format of public key certificates. X.509 certificates are used in many Internet protocols, including TLS/SSL, the basis for HTTPS. They are also used in offline applications, like electronic signatures. An X.509 certificate contains a public key and an identity, and is either signed by a certificate authority or self-signed. X.509 is defined by the International Telecommunications Union’s “Standardization Sector” (ITU-T), and based on ASN.1, another ITU-T standard. † ℹ︎ itu.int/rec/T-REC-X.509
x-height : The distance between the baseline and the mean line of lowercase letters in a typeface. Typically, this is the height of the letter “x” in a font (the source of the term), as well as the letters “v”, “w”, and “z”. (Curved letters such as “a”, “c”, “e”, “m”, “n”, “o”, “r”, “s”, and “u” tend to exceed the x-height slightly, due to overshoot.) As one of the most important dimensions of a font, x-height is used to define how high lowercase letters are compared to uppercase letters. †
XAML : → Extensible Application Markup Language
XForms : An XML format used for collecting inputs from web forms. XForms was designed to be the next generation of HTML forms, but is generic enough that it can also be used in a standalone manner or with presentation languages other than XHTML to describe a user interface and a set of common data manipulation tasks. XForms 1.0 (Third Edition) was published in 2007. † ℹ︎ w3.org/TR/xforms
XHP : An augmentation of PHP and Hack developed at Facebook to allow XML syntax for the purpose of creating custom and reusable HTML elements. XHP was first released in 2010. †
XHR : → XMLHttpRequest
XHTML : → Extensible HyperText Markup Language
XLink : An XML markup language and W3C specification that provides methods for creating internal and external links within XML documents, and associating metadata with those links. † ℹ︎ w3.org/TR/xlink11
XML : → Extensible Markup Language
XML Schema Definition : A specification for how to formally describe the elements in an Extensible Markup Language (XML) document. XSD can be used by programmers to verify each piece of item content in a document, and to check if it adheres to the description of the element it is placed in. † ℹ︎ w3.org/TR/xmlschema-0
XML User Interface Language : A user interface markup language developed by Mozilla. XUL is implemented as an XML dialect, enabling graphical user interfaces to be written in a similar manner to web pages. Such applications must be created using the Mozilla code base (or a fork of it), with the Firefox web browser being the most prominent example. † ℹ︎ developer.mozilla.org/XUL
XML-RPC : A remote procedure call (RPC) protocol which uses XML to encode its calls and HTTP as a transport mechanism. XML-RPC was created in 1998 by Dave Winer. As new functionality was introduced, the standard soon evolved into what is now SOAP. † ℹ︎ xmlrpc.com
XMLHttpRequest : An API in the form of an object whose methods transfer data between a web browser and a web server. The object is provided by the browser’s JavaScript environment. Particularly, retrieval of data from XHR for the purpose of continually modifying a loaded web page is the underlying concept of AJAX design. Despite the name, XHR can be used with protocols other than HTTP and data can be in the form of not only XML, but also HTML, JSON, or plain-text. †
XP : → Extreme Programming
XPath : A query language for selecting nodes from an XML document. In addition, XPath may be used to compute values (e.g., strings, numbers, or Boolean values) from the content of an XML document. XPath was developed in 1998 by the World Wide Web Consortium (W3C). † ℹ︎ w3.org/TR/xpath-3
XPointer : A system for addressing components of XML-based Internet media. It is divided among four specifications: a “framework” that forms the basis for identifying XML fragments, a positional element addressing scheme, a scheme for namespaces, and a scheme for XPath-based addressing. XPointer, or the XPointer Framework, became a W3C Recommendation in 2003. † ℹ︎ w3.org/TR/xptr-framework
XQuery : A query and functional programming language that queries and transforms collections of structured and unstructured data, usually in the form of XML or text, and with vendor-specific extensions for other data formats (JSON, binary, etc.). The language is developed by the XML Query Working Group of the World Wide Web Consortium (W3C). The work is closely coordinated with the development of XSLT by the XSL Working Group; the two groups share responsibility for XPath, which is a subset of XQuery. † ℹ︎ w3.org/TR/xquery-31
XSD : → XML Schema Definition
XSL : → Extensible Stylesheet Language
XSLT : → Extensible Stylesheet Language Transformations
XSRF : → Cross-Site Request Forgery
XSS : → Cross-Site Scripting
XSSI : → Cross-Site Script Inclusion
XUL : → XML User Interface Language
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